1. Field of the Invention
The invention relates to optical communications systems using microstructured optical fiber.
2. Discussion of the Related Art
Microstructured optical fiber, e.g., fibers containing capillary air holes, are known. Such fibers have experienced renewed interest due to a variety of interesting properties observed, including supercontinuum generation and soliton generation. See, e.g., B. J. Eggleton et al., xe2x80x9cCladding-Mode-Resonances in Air-Silica Microstructure Optical Fibers,xe2x80x9d Journal of Lightwave Technology, Vol. 18, No. 8 (2000); J. C. Knight et al., xe2x80x9cAnomalous Dispersion in Photonic Crystal Fiber,xe2x80x9d IEEE Photonics Technology Letters, Vol. 12, No. 7 (2000); J. Ranka et al., xe2x80x9cVisible continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm,xe2x80x9d Optics Letters, Vol. 25, No. 1 (2000); and U.S. Pat. Nos. 5,907,652 and 6,097,870. While these fibers have generated interesting and attractive properties, several practical difficulties exist. For example, many of these unique properties are found in microstructured fiber that have an extremely small core. Coupling light efficiently into such a fiber is thus a significant hurdle. In addition, the robustness of such fibers is sometimes in question. A variety of similar hurdles stand between the current state of the art and commercial feasibility.
Thus, improvements in fabrication and design of systems utilizing microstructured optical fiber, as well a new ways to develop and manipulate the fiber itself, are desired.
The invention relates to improved techniques for utilizing microstructured optical fibers in a variety of systems, e.g., techniques for manipulating microstructured fiber, forming robust small-diameter microstructured fiber, and/or for manipulating modes propagating through a microstructured fiber. The process is relatively straightforward and also provides for efficient incorporation and operation of the resultant fiber in a communications system.
Stated generally, the invention involves providing a microstructured fiber having a core region, a cladding region, and one or more axially oriented elements (e.g., capillary air holes) in the cladding region. A portion of the microstructured fiber is then treated, e.g., by heating and stretching the fiber, such that at least one feature of the fiber microstructure is modified along the propagation direction, e.g., the outer diameter of the fiber gets smaller, the cross-section of the axially oriented elements get smaller, or the axially oriented elements collapse. The treatment is selected to provide a resultant fiber length that exhibits particular properties such as mode contraction (optionally leading to soliton generation) or mode expansion. Advantageously, the process is performed such that the resultant fiber length is able to readily be coupled to a standard transmission fiber, i.e., the core sizes are similar, which allows efficient coupling of light. (Modified along the propagation direction indicates that as one moves along the propagation direction, one or more aspects of the microstructure vary. Outer diameter indicates the diameter of the outermost cladding, not including any protective coating. Fiber length, as used herein, indicates the entire structure, i.e., untreated and treated fiber sections.) Fiber lengths fabricated according to the invention are useful for a variety of applications, including dispersion management, optical regeneration, reshaping, and retiming, providing nonlinear effects such as soliton generation, soliton self-frequency shift, and pulse compression, and providing efficient coupling into laser diodes and similar devices.
In one embodiment, reflected in FIGS. 1A and 1B, the treated portion of the fiber 10 is heated and stretched to form at least one tapered region 22, 23 and a waist region 24, the tapered region(s) leading from an untreated portion of the fiber to the waist region. Typically, the microstructure of the fiber is maintained in the at least one tapered region, and in at least a portion of the waist region. In fact, one way to readily fabricate sections of small-core microstructured fiber of the type discussed above is to stretch a larger profile microstructured fiber. And, moreover, because a portion of the fiber retains its original core diameter, light from an adjacent transmission fiber is capable of being efficiently coupled into the microstructured fiber length. The resultant structure is, for example, capable of highly advantageous soliton self-frequency shift, discussed in more detail below. (Maintenance of the microstructure in the tapered region and/or the waist region, as used herein, indicates that the presence and arrangement of at least a portion of the axially oriented elements are maintained, although likely in a different size and proportion than in the initial fiber; it is possible that a doped core present in the initial fiber will essentially disappear in the waist region, but such an effect does not indicate that the microstructure has not been maintained.)
Significantly, the above embodiment is typically performed such that light propagating through the waist region fiber is confined by the axially oriented elements, e.g., by the capillary air holes. Confined, as used herein, indicates confinement due to the effective refractive index profile provided by the combination of the silica and the axially oriented elements, or due to a bandgap effect provided by periodically disposed axially oriented elements. In this respect, the invention constitutes a significant improvement over prior art systems that used tapering techniques. For example, in T. A. Birks et al., xe2x80x9cGeneration of an ultra-broad supercontinuum in tapered fibres,xe2x80x9d CLEO""00, postdeadline paper CPD30 (2000), the authors report tapering a conventional optical fiber, i.e., a non-microstructured fiber, down to a waist region having a diameter less than 2 xcexcm. At this small diameter, the core essentially disappears, and the entire fiber diameter then constitutes a core region with the cladding provided by the surrounding air. While unique properties of the type discussed above were exhibited by the stretched fiber, the fact that the surrounding air provides the cladding renders the design commercially unfeasible. Specifically, not only does the waist region becomes highly sensitive to bendingxe2x80x94i.e., bends introduce loss, but a polymer re-coat of the waist region modifies the air-silica boundary and thereby similarly introduces loss. Thus, the system described by Birks et al. is not robust enough for feasible commercial use.
In contrast, according to the above embodiment of the invention, the axially oriented elements present in the waist region fiber confine propagating light therein, i.e., provide an effective cladding. Because the outside of the fiber is therefore not functioning as the cladding, the resultant waist region is highly robust. Specifically, since the exterior air is not used as cladding, a larger diameter can be provided, i.e., there can be additional silica surrounding the axially oriented elements, which improves the robustness of the waist region. Moreover, also in contrast to the Birks et al. fiber, the fiber of the invention is able to endure bending as well as a polymer re-coat, further increasing the ease with which the overall fiber length is able to be incorporated into a system.
In another embodiment of the invention, reflected in FIG. 2, the microstructure of the initial fiber is treated such that axially oriented elements (typically capillary air holes) are partially or fully collapsed in at least part of the treated portion of the fiber, while the overall diameter of the treated section remains about the same as the untreated section (e.g., the diameter of the treated portion is generally at least 90% of its original diameter). (Partial collapse indicates a reduction in cross-section of the elements, but with the elements still intact.) This gradual collapse as one moves along the propagation direction is able to provide mode expansion, i.e., the effective refractive index profile provided by the presence of the axially oriented elements disappears, leaving a silica cladding in its place. Such mode expansion is useful, for example, in a variety of applications were conversion into a larger mode is desired, e.g., to minimize coupling losses between a fiber and another element such as a larger area detector or a laser diode (see, e.g., W. T. Chen and L. A. Wang, xe2x80x9cLaser-to-fiber coupling scheme by utilizing a lensed fiber integrated with a long-period grating,xe2x80x9d IEEE Photonics Tech. Lett., Vol. 12, No. 5, 501-503 (2000)). Such gradual collapse is also possible in combination with some stretching. For example, it is possible to take a microstructured fiber, collapse the air holes at one end, which enhances splicing and coupling to standard transmission fiber, and stretch another portion of the fiber to provide desired properties.